The really first big upgrade of the AF final drives happened with the 21XX series, when they became very strong.
Earlier, they did small upgrades, but did not reall get it fixed untill the much heavier drives on the 21 series. Putting 21 series final drives on a 14 or 16 series combine is a popular upgrade if you can find them.
The drive cases were strengthened several times with added ribbing, plus the sheetmetal case covers were replaced with much more rigid cast iron covers which helped the case spreading problem and extended wheel bearing life. The shaft breakage problem was pretty well eliminated with the stronger "smooth back" axle shafts.
One tip, ANY axle seal leakage means a 99% chance that the outer wheel bearing is failing and needs to be fixed RIGHT NOW. Continuing on can turn a $300 bearing replacement into a $2500 destroyed final drive disaster.